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Biochar has gained significant attention in agricultural and environmental research over the last two decades. This comprehensive review evaluates the effects of biochar on soil organic carbon (SOC), emission of non-CO2 greenhouse gases, and crop yield, including related mechanisms and major influencing factors. The impacts of biochar on SOC, methane and nitrous oxide emissions, and crop yield are controlled by biochar and soil properties and management practices. High-temperature biochar produced from lignin-rich feedstocks may decrease methane and nitrous oxide emissions in acidic soils and strengthen long-term carbon sequestration due to its stable aromatic structure. In contrast, low-temperature biochar from manure may increase crop yield in low-fertility soils. Applying biochar to farmlands in China can increase SOC content by 1.9 Pg C and reduce methane and nitrous oxide emissions by 25 and 20 Mt CO2-eq year−1, respectively, while increasing crop yields by 19%. Despite the increasing evidence of the positive effects of biochar, future research needs to explore the potential factors that could weaken or hinder its capacity to address climate change and secure crop production. We conclude that biochar is not a universal solution for global cropland; however, targeted applications in fields, landscapes, or regional scales, especially in low fertility and sandy soils, could realize the benefits of biochar as a climate-smart measure. Highlights The findings of research on biochar's effects on soil C sequestration, GHG mitigation, and crop production were summarized. The factors influencing the impact of biochar on soil functioning were reviewed. The effects of biochar on soil C sequestration and GHG mitigation in farmlands of China were quantified. [ABSTRACT FROM AUTHOR]

Tropical and subtropical acidic soils have been well documented as hotspots of global soil nitrogen (N) oxide (i.e., nitrous oxide (N2O) and nitric oxide (NO) emissions). While the effectiveness of possible mitigation options has been extensively examined in croplands, little is known about their effectiveness in reducing N‐oxide emissions from acidic soils of rapidly expanding tea plantations in China. Here, we conducted a 2‐year field experiment to investigate how organic substitution for synthetic fertilizer and biochar amendment affect soil N‐oxide emissions from a subtropical tea plantation. Across the 2‐year measurement period, full organic substitution for synthetic fertilizer significantly increased N2O emissions by an average of 17% while had a lower NO emission compared to synthetic fertilizer alone. Our global meta‐analysis further revealed that full or partial organic fertilizer substitution resulted in a 29% (95% confidence interval: 5%–60%) increase of N2O emissions from acidic soils. In contrast, irrespective of fertilizer type, biochar amendment significantly reduced N2O emissions by 14% in the first but not second experimental year, suggesting a transient effect. The trade‐off effect of full organic substitution on N2O and NO emissions may be attributed to the favorable conditions for N2O production due to the stimulated activity of nitrifiers and denitrifiers. The suppression of N2O emission following biochar amendment was probably due to promoted further reduction of N2O to dinitrogen. The fertilizer‐induced emission factor (EF) of N2O (2.1%) in the tea plantation was greater than the current IPCC default value, but the EF of NO (0.8%) was comparable to the global estimate. Taken together, while biochar amendment could have mitigation potential, cautions are needed when applying organic substitution for synthetic fertilizer as mitigation options for acidic soils as hotspots of N‐oxide emissions. [ABSTRACT FROM AUTHOR]

Earthworms and biochar tend to have opposite effects on heavy metal bioavailability in soil. However, the influence and controlling process of earthworms on the immobilisation effect of biochar remain poorly understood. Through the co-cultivation of earthworms with rice-husk biochar and sludge biochar in heavy metal-contaminated soil and desorption experiments involving simulated earthworm gut, we explored the factors that earthworms influence the heavy metal immobilisation ability of biochar. Our results showed that rice-husk biochar and sludge biochar effectively immobilized heavy metals in soil, whereas earthworm activity mobilised heavy metals in biochar-treated soil, which weakens the immobilisation of biochar. The soil pH reduction effect of earthworms by increasing the abundance of soil ammonia-oxidising bacteria to promote soil nitrification is an important mechanism through which earthworms mobilise heavy metals; however, this process did not occur within 10 days of incubation. Nitrification inhibitors effectively inhibit the mobilisation of heavy metals in soil by earthworms. In addition, the bioavailability of heavy metals in earthworm casts was significantly higher than those in the surrounding soil and earthworm-free soil. Moreover, simulated earthworm gut fluid promoted the re-release of heavy metals from the soil and biochar particles. These results suggest that the gut digestion of earthworms is another important mechanism by which earthworms mobilise soil heavy metals and weaken the immobilisation of biochar. Therefore, earthworms weakened the immobilisation effect of biochar mainly by promoting nitrification to reduce soil pH and through gut digestion. [Display omitted] • Earthworms and biochar have opposite effects on heavy metal bioavailability in soil. • Earthworm activity weakens the immobilisation effect of biochar on heavy metals. • Reduction of soil pH by earthworms weakens immobilisation of biochar. • Earthworms promote soil nitrification to reduce soil pH by increasing AOB abundance. • Earthworm gut digestion is another way to weaken the immobilisation of biochar. [ABSTRACT FROM AUTHOR]

• Soil pH and climate variables are key factors influencing N2O emissions. • We developed a nonlinear empirical model of N2O emissions applicable to tea plantations. • Tea plantations are hotspots for N2O emissions from agricultural sector with 46.5 Gg N yr-1. • The proposed abatement strategy can reduce about 1/3 of the total global emissions. Estimating N 2 O emissions from the agricultural sector and developing effective reduction strategies are essential to achieving the Paris Agreement 2 °C target. Based on 3705 observations from 435 articles, we demonstrated that the response of N 2 O emissions was more sensitive to N inputs on acidic soils than alkaline soils and that climatic factors influence this difference. Total global N 2 O emissions from tea plantations in the 2010s were estimated to be 46.5 Gg N yr–1 using an exponential model developed herein. Tea plantations are a significant contributor to N 2 O emissions from the agricultural sector in several countries. The intensity of yield-scale GHG emissions from tea was significantly higher than in other upland cereals. Applying climate-smart practices in Chinese tea plantations could reduce emissions equivalent to one-third of the global total. We conclude that accurate identification of N 2 O emission hotspots and implementation of targeted measures are essential to achieving global temperature control targets. [Display omitted] [ABSTRACT FROM AUTHOR]

• An incubation experiment and a meta -analysis were conducted. • Soil pH significantly influenced the efficiency of DMPP. • DMPP performed poorer in acidic tea plantation soils than in cropland soils for N 2 O mitigation. • Soil pH regulated the AOB- and heterotrophic nitrification-derived N 2 O production. • Heterotrophic nitrification-derived N 2 O production in acidic soils cannot be ignored. The emission pathways and intensities of nitrous oxide (N 2 O) vary across land uses, and the efficacy of mitigation measures may also differ. To investigate the key factors influencing the effectiveness of the nitrification inhibitor 3,4-Dimethylpyrazole phosphate (DMPP) in mitigating N 2 O under various land use conditions, we collected ten pairs of soils from tea plantations and adjacent cultivated land for laboratory incubation. As a complementary study, we conducted a meta -analysis based on 261 pairs of observations from 40 incubation studies to investigate the determinants of soil N 2 O mitigation by DMPP and to validate our experimental results. Results of the incubation experiment showed that DMPP was significantly less effective in mitigating N 2 O in tea plantation soils (73%) than in cropland soils (82%). Soil pH is a key factor influencing DMPP efficiency under different land use conditions. The lower pH of acidic tea plantation soils resulted in lower abundance and activity of ammonia-oxidizing bacteria (AOB). Under these low pH conditions, heterotrophic nitrification became an important source of N 2 O emissions in acidic tea plantation soils. As DMPP primarily inhibits AOB, it was less effective against heterotrophic nitrification, which likely contributed to its reduced efficacy in tea plantation soils compared to cropland soils. Our meta -analysis further confirmed the critical role of soil pH in determining DMPP effectiveness. The effect of soil pH on DMPP efficacy was mainly attributed to its regulation of heterotrophic and AOB-derived nitrification. These findings suggest that the field efficacy of DMPP in N 2 O abatement in tea plantations requires further investigation. Given the significant contribution of heterotrophic nitrification to N 2 O emissions in strongly acidic tea plantation soils, alternative N 2 O mitigation strategies should be explored. [ABSTRACT FROM AUTHOR]

Plant growth-promoting microbes (PGPMs) are documented to stimulate nitrification rates and reduce N 2 O emissions in acidic soils. These microbes play a role in the nitrogen (N) transformation process, although the specific functions and mechanisms by which they affect the gross N transformation are not well understood. In particular, the influence of PGPMs on the relative predominance of ammonia oxidizers in the nitrification process is still unclear. In this study, we conducted a 15N tracing experiment to reveal the impact of PGPM Bacillus velezensis SQR9 on gross N transformations in acidic soils, as well as the microbial pathways involved. SQR9 inoculation considerably enhanced the processes of soil gross mineralization and nitrification by 14.6 % and 29.5 %, respectively. This improvement was found to be associated with the soil's dissolved organic carbon (DOC) content and carbon-to‑nitrogen (C/N) ratio. SQR9 increased the abundance of ammonia-oxidizing bacteria (AOB), resulting in a substantial promotion of autotrophic nitrification, which occupied a dominant role (71.3–82.6 %) in the nitrification process. SQR9 significantly stimulated the proportion of AOB, indicating a transition from ammonia-oxidizing archaea (AOA) to AOB as the dominant ammonia oxidizers, hence promoting the gross nitrification rate. In conclusion, the heightened rates of N transformation are highly associated with the modification of the ammonia-oxidizer B. velezensis SQR9. Our findings offer an updated insight into how PGPMs cause N transformation and provide a theoretical basis for the sensible application of PGPMs in agricultural development. • SQR9 increases gross nitrification and mineralization rates in acidic soils. • SQR9 stimulates soil DOC and suppresses the C/N ratio to promote N transformation. • Autotrophic nitrification plays a dominant role in the nitrification process. • SQR9 increases the AOB contribution to nitrification to promote gross N transformation. [ABSTRACT FROM AUTHOR]

Biochar amendment has been shown to reduce nitrous oxide (N 2 O) emissions from acidic soils in tea plantations. Given that both soil bacterial and fungal denitrifications can produce N 2 O, their relative contributions to the decrease in N 2 O emissions following biochar amendment remain unclear. Here, we examined soils sampled from a subtropical tea plantation that had received 2 years of biochar amendment. Measurements of the relative contributions of fungi and bacteria to N 2 O production were taken by the substrate-induced respiration method implemented with the addition of selective inhibitors. The abundances of total fungi, bacteria, and key N 2 O-related bacterial genes were quantified by q-PCR, and the composition of the fungal community was analyzed by 18S rRNA amplicon sequencing. The results showed that the contribution of fungi to N 2 O production (52%) was greater than that of bacteria (18%) for the N-applied acidic soils. Biochar amendment significantly decreased the fungal abundances and the fungal contribution to N 2 O production (by 28%). In contrast, biochar amendment significantly increased the abundances of N 2 O-related bacteria (e.g., ammonia-oxidizing bacteria (AOB), nirS , nosZ). Structural equation models (SEMs) revealed that biochar decreased the fungal contribution to N 2 O production through enhancing the soil pH and shifting the fungal community composition. Our results highlight that the decreased N 2 O emissions could be ascribed to the stimulated N 2 O consumption process (driven by N 2 O-consuming bacteria encoded by the nosZ gene) and suppressed fungal dominance in acidic soils from tea plantations. This study presents relatively comprehensive insights into the regulatory roles of biochar on soil microbe-mediated N 2 O production processes. • Fungi played a more important role than bacteria in N 2 O production from acidic soils. • Biochar shifted the fungal community on genus level from acidic soils. • Biochar decreased the fungal contribution to N 2 O production. [ABSTRACT FROM AUTHOR]

• Biochar amendment decreased N 2 O and NO emissions from acidic soils. • Biochar amendment stimulated both nitrification and denitrification processes. • Soil N 2 O was produced mainly through denitrification in acidic soils. • Soil DOC and pH were key factors to N 2 O and NO emissions from acidic soils. • Soil N 2 O emissions were negatively related to nosZ gene abundances. Acidic soils are hotspots of nitrous oxide (N 2 O) and nitric oxide (NO) and biochar is documented to have the potential for mitigating N 2 O and NO. The N 2 O and NO emissions associated with soil functional genes and physicochemical properties under biochar amendment remains unclear in acidic soils. Here, we carried out a two-year field study to examine the responses of soil N 2 O and NO emissions to biochar amendment in a subtropical tea plantation in China. Measurements of N 2 O and NO fluxes were taken from inter-row soils using the static chamber method. We also measured the seasonal changes in soil key nitrogen (N)-cycling functional genes and physicochemical properties. Annual N 2 O and NO emissions averaged 27.31 kg N 2 O-N ha−1 yr−1 and 8.75 kg NO-N ha−1 yr−1 for the N fertilizer applied plots, which were decreased by 24% and 16% due to biochar application, respectively. In addition, both potential nitrification (PNR) and denitrification (PDR) rates were stimulated by biochar amendment, which significantly increased the abundances of bacterial amoA (AOB), nirK and nosZ genes. Changes in the composition of the N 2 O-related microbial functional community were closely associated with soil PNR, pH, DOC, and NO 3 −-N contents. The ratios of NO/N 2 O were mainly lower than 1, suggesting that N 2 O was produced mostly through denitrification rather than nitrification. There were negative correlations between soil N 2 O and NO emissions and soil PDR and pH, and soil N 2 O emissions were negatively correlated with nosZ gene abundances. Together, the decrease in N 2 O and NO emissions following biochar application could be largely attributed to the enhanced denitrification process, in which biochar enriched the nirK and nosZ genes abundance, resulting from the enhancement of soil DOC and pH in acidic soils. [ABSTRACT FROM AUTHOR]